Consider adding additional relations from GO

[stuppie]

Details of all relations and usage examples
AnnotationExtensionRelations

Examples:
cannabinoid signaling pathway -> has_input -> cannabinoid (See neighborhood tab)
cannabinoid signaling pathway -> starts_with -> cannabinoid receptor activity
trans-synaptic signaling by nitric oxide -> mediated_by -> nitric oxide

[andrewsu]

Just a note that we should wait for the conclusion of the GO licensing discussion (initiated at the GO meeting last week) before expanding the scope of what we're importing.